This invention relates in general to casting of metallic strands and more specifically to a cooled mold assembly and withdrawal process for the continuous, high-speed casting of strands of copper and copper alloys including brass.
It is well known in the art to cast indefinite lengths of metallic strands from a melt by drawing the melt through a cooled mold. The mold generally has a die of a refractory material such as graphite cooled by a surrounding water jacket. U.S. Pat. No. 3,354,936 for example, describes a cooled mold assembly sealed into the bottom wall of the melt container to downcast large billets. The force of gravity feeds the melt through the mold. In downcasting, however, there is a danger of a melt "break out" and the melt container must be emptied or tilted to repair or replace the mold or the casting die.
Horizontal casting through a chilled mold has also been tried. Besides the break out and replacement problems of downcasting, gravity can cause a non-uniform solidification resulting in a casting that is not cross-sectionally uniform or having an inferior surface quality.
Finally, various arrangements have been used for upcasting. Early efforts are described in U.S. Pat. No. 2,553,921 to Jordan and U.S. Pat. No. 2,171,132 to Simons. Jordan employs a water cooled, metallic "mold pipe" with an outer ceramic lining that is immersed in a melt. In practice, no suitable metal has been found for the mold pipe, the casting suffers from uneven cooling, and condensed metallic vapors collect in a gap between the mold pipe and the liner due to differences in their coefficients of thermal expansion. Simons also uses a water-cooled "casing" but it is mounted above the melt and a vacuum is required to draw melt up to the casing. A coaxial refractory extension of the casing extends into the melt. The refractory extension is necessary to prevent "mushrooming," that is, the formation of a solid mass of the metal with a diameter larger than that of the cooled casing. As with Jordan, thermally generated gaps, in this instance between the casing and the extension, can collect condensed metal vapors which results in poor surface quality or termination of the casting.
U.S. Pat. Nos. 3,746,077 and 3,872,913 describe more recent upcasting apparatus and techniques. The '913 patent avoids problems associated with thermal expansion differences by placing only the tip of a "nozzle" in the melt. A water-cooled jacket encloses the upper end of the nozzle. Because the surface of the melt is below the cooling zone, a vacuum chamber at the upper end of the nozzle is necessary to draw the melt upwardly to the cooling zone. The presence of the vacuum chamber however limits the rate of strand withdrawal and requires a seal.
The '077 patent avoids the vacuum chamber by immersing a cooling jacket and a portion of an enclosed nozzle into the melt. The immersion depth is sufficient to feed melt to the solidification zone, but it is not deeply immersed. The jacket as well as the interfaces between the jacket and the nozzle are protected against the melt by a surrounding insulating lining. The lower end of the lining abuts the lower outer surface of the nozzle to block a direct flow of the melt to the cooling jacket.
The foregoing systems are commonly characterized as "closed" mold in that the liquid metal communicates directly with the solidification front. The cooled mold is typically fed from an adjoining container filled with the melt. In contrast, an "open" mold system feeds the melt, typically by a delivery tube, directly to a mold where it is cooled very rapidly. Open mold systems are commonly used in downcasting large billets of steel, and occassionally aluminum, copper or brass. However, open mold casting is not used to form products with a small cross section because it is very difficult to control the liquid level and hence the location of the solidification front.
A problem that arises in closed mold casting is a thermal expansion of the bore of the casting die between the beginning of the solidification front and the point of complete solidification termed "bell-mouthing." This condition results in the formation of enlargements of the casting cross section which wedge against a narrower portion of the die. The wedged section can break off and form an immobile "skull." The skulls can either cause the strand to terminate or can lodge on the die and produce surface defects on the casting. Therefore it is important to maintain the dimensional uniformity of the die bore within the casting zone. In the '913 and '077 systems, these problems are controlled by a relatively gentle vertical temperature gradient along the nozzle due in part to a modest cooling rate to produce a generally flat solidification front. With this gentle gradient, acceptable quality castings can be produced only at a relatively slow rate, typically five to forty inches per minute.
Another significant problem in casting through a chilled mold is the condensation of metallic vapors. Condensation is especially troublesome in the casting of brass bearing zinc or other alloys bearing elements which boil at temperatures below the melting temperature of the alloy. Zinc vapor readily penetrates the materials commonly used to form casting dies as well as the usual insulating materials and can condense to liquid in critical regions. Liquid zinc on the die near the solidification front can boil at the surface of the casting resulting in a gassy surface defect. Because of these problems, present casting apparatus and techniques are not capable of commercial production of good quality brass strands at high speeds.
The manner in which the casting is drawn through the chilled mold is also an important aspect of the casting process. A cycled pattern of a forward withdrawal stroke followed by a dwell period is used commercially in conjunction with the mold unit described in the aforementioned U.S. Pat. No. 3,872,913. U.S. Pat. No. 3,908,747 discloses a controlled reverse stroke to form the casting skin, prevent termination of the casting, and compensate for contraction of the casting within the die as it cools. British Pat. No. 1,087,026 also discloses a reverse stroke to partially remelt the casting. U.S. Pat. No. 3,354,936 discloses a pattern of relatively long forward strokes followed by periods where the casting motion is stopped and reversed for a relatively short stroke. This pattern is used in downcasting large billets to prevent inverse segregation. In all of these systems, however, the stroke velocities and net casting velocities are slow. In the '936 system, for example, forward strokes are three to twenty seconds in duration, reverse strokes are one second in duration, and the net velocity is thirteen to fifteen inches per minute.
It is therefore a principal object of this invention to provide a mold assembly and method for the continuous casting of high quality metallic strands and particularly those of copper and copper alloys including brass at production speeds many times faster than those previously attainable with closed mold systems.
Another object of the invention is to provide such a cooled mold assembly for upcasting with the mold assembly immersed in said melt.
A further object is to provide such a mold assembly that accommodates a steep temperature gradient along a casting die, particularly at the lower end of a solidification zone, without the formation of skulls or loss of dimensional uniformity in the casting zone.
Still another object is to provide a casting withdrawal process for use with such a mold assembly to produce high quality strands at exceptionally high speeds.
A further object is to provide a mold assembly with the foregoing advantages that has a relatively low cost of manufacture, is convenient to service and is durable.